Unfortunately eager loading polymorphic associations isn't actually possible. The polymorphic association holds a a class name and an id present on that class. The polymorphic associations, though, don't have a way to join them in a SQL query since SQL doesn't support any sort of meta-programing.

For instance:

Address is kept in addresses table, and has an addressable reference. In the database there would be "addressable_id" and "addressable_type". There's no way to use "addressable_type" to make an INNER JOIN in the SQL query.